Conventionally, a communication system configured as follows has been known. The communication system is configured with a server that transmits data regarding services that the server can provide to a client in response to a request for such data from the client. Then, data is transmitted from the client to the server that has been generated by the client based on the data regarding the services. Then, the server processes the transmitted data subject to the service, thereby providing the service.
An example of conventionally known techniques can be found at a web site entitled “RSS Reader Rabbit Ticker” at <http://www.work-at.co.jp> (the site exists as of Oct. 20, 2004). In this site, a client device is disclosed that periodically transmits inquiries to the server for news information. When there is news information, a client can access the server to obtain the news information.
Another example can be found in Japanese Patent Provisional Publication No. HEI 9-238215 (hereinafter, referred to as '215 publication). According to the '215 publication, there are a digital copier and a host computer, both connected to a network. A function that is not implemented in the digital copier is implemented in the host computer. According to the '215 publication, control command codes and/or image data such as image density data is transmitted, through the network, from the digital copier to the host computer. Then, the host computer analyzes the received control commands, and then applies image processing to the received image data using a function designated by the received (and analyzed) command codes. The image data thus processed is then transmitted from the host computer to the digital copier through the network. The digital copier then prints the thus received processed image data. As a result, the digital copier utilizes the service provided by the host computer for processing the image data.
Further, in the system disclosed in '215 publication, when new functions are implemented in the host computer, information regarding the new functions is transmitted from the host computer to the digital copier. Then, the user can select, from among the new functions, only the functions that the user intends to register with the copier.
However, the system disclosed in '215 publication is not applicable to an environment where the services to be provided are increased/decreased irregularly. That is, according to such a system, when a new function is added to the host computer, it can be used only when the function is registered with the digital copier. However, as with a server on the Internet, if a variety of services are frequently and irregularly increased/decreased, a registration operation to the digital copier is very troublesome and the amount of such information is considerably large. Therefore, the technique disclosed in the '215 publication may not be suitable for a system utilizing the Internet.
Incidentally, the service providing system as described above is typically configured such that the digital copier, or a client, which is connected with a LAN (Local Area Network), and the LAN is connected to a WAN (Wide Area Network) such as the Internet through a router. The router is used for improving network traffic efficiency, and for strengthening security of the system. In such a network employing a router, unnecessary packets will not go out of the LAN, and unnecessary packets will not come in the LAN from outside.
Generally, according to the above configuration, the LAN accepts data which is transmitted as a response to a request that has been transmitted from a device inside the LAN. In such a network, it is impossible to receive data which has been positively transmitted from a server outside the LAN without a request from a device inside the LAN. That is, to any device of a network that is connected to the WAN (e.g., the Internet) through a router, the server on the WAN cannot positively provide a service.
Thus, when there is a service the server can provide to the client connected to the LAN, a system employing a polling communication may be usable. Such a system may be very effective particularly when new services are constantly provided to the client.
However, in such a system, if the number of the services to be provided by the server increases and/or when the number of clients increases, the amount of data to be transmitted using the polling communication (the amount of inquiry data) increases, which increases the network traffic. Further, since access by the client increases, a relatively large burden may be applied to the server.